1. Field of the Invention
The present invention relates to amass spectrometry method, in which a sample in contact with a surface of a substrate for mass spectrometry is irradiated with measurement light to desorb an analyte contained in the sample from the surface of the substrate for mass spectrometry, and mass spectrometry is performed on the desorbed analyte. Further, the present invention relates to a mass spectrometry apparatus used in the mass spectrometry method.
2. Description of the Related Art
As mass spectrometry for identifying a substance or the like, a mass spectrometry method in which a sample in contact with a substrate for mass spectrometry is irradiated with measurement light to desorb an analyte from the substrate for mass spectrometry and the desorbed analyte is detected separately for each mass is known. For example, in a time-of-flight mass spectrometry (Time of Flight Spectroscopy: TOF-MS), a substance desorbed from a substrate is caused to fly for a predetermined distance, and the mass of the substance is analyzed based on the time period of flight of the substance.
In such mass spectrometry methods, the analyte is ordinarily ionized and desorbed. However, when the analyte is a substance that is hard to evaporate, such as an organism substance, or a high molecular weight substance, such as a synthetic high polymer (macromolecule), it is difficult to ionize and desorb the analyte. Therefore, various methods are studied to make it possible to perform mass spectrometry on such substances.
Especially, in mass spectrometry of a substance that is hard to evaporate, a biomolecule, a high molecular weight substance, such as a synthetic high polymer, or the like, a matrix-assisted laser desorption/ionization (MALDI) mass spectrometry is widely used as a soft method, which induces a relatively small chemical change, such as fragmentation and denaturation, in an analyte. In the MALDI, an analyte is mixed into sinapinic acid, glycerin or the like, which is called as a matrix, to form a mixed crystal. The mixed crystal is used as a sample, and the analyte is vaporized together with the matrix by using light energy absorbed by the matrix. Further, the analyte is ionized by movement of protons between the matrix and the analyte (please refer to Japanese Unexamined Patent Publication No. 9 (1997)-320515 and the like). However, when the analyte is a synthetic high polymer or the like, solubility to a solvent, the polarity of a polymer chain and the like greatly differ according to a difference in the chemical structure of the polymer chain. Further, even if the main chain structure is the same, various properties of the analyte differ according to a difference in an average molecular weight, the chemical structure of an end group, and the like. Therefore, it is necessary to optimize the kind of a matrix material and the method for preparing the crystal based on the kind of the analyte.
Further, a surface-assisted laser desorption/ionization-mass spectrometry (SALDI-MS) is being studied. In SALDI-MS, no matrix material is used, and a function for assisting desorption and ionization of the analyte is provided in the substrate for mass spectrometry per se to carry out soft ionization. For example, in the specification of U.S. Patent Application Publication No. 20080073512 and the specification of U.S. Patent Application Publication No. 20060157648, soft ionization is carried out by utilizing mutual reaction between a silicon nanostructure and measurement light in a substrate for mass spectrometry that adopts a porous silicon substrate, which has a nano-order porous structure on the surface thereof.
Further, Japanese Unexamined Patent publication No. 2005-098909 discloses an ionization apparatus in which desorption and ionization of an analyte is assisted by near-field light generated at the leading end of a metal probe by irradiation with measurement light, and a mass spectrometry apparatus using the ionization apparatus.
However, in the mass spectrometry apparatuses using the SALDI-MS and the metal probe, enhancement of the ion detection efficiency is insufficient. Therefore, when mass spectrometry is carried out on a substance that is hard to evaporate or a high molecular weight substance, high power measurement light is needed. Hence, problems, such as fragmentation and denaturation of the analyte, a drop in the S/N ratio by desorption of a substance contained in a portion of the sample in the vicinity of an irradiated portion, and a deformation of the substrate per se, remain unsolved.